This invention relates to the separation of higher molecular weight components from lower molecular weight components in a fluid stream. In a specific embodiment, it relates to the separation of the ethane and higher molecular weight components from a natural gas stream containing methane.
Natural gas as it comes from the ground generally is not suitable for use directly without some processing. The basic processing operations carried out in a natural gas plant are to first remove acid gases such as CO.sub.2 and H.sub.2 S and then to pass the gas through a dehydration means to remove water. The resulting product can then be used as a fuel. However, such streams generally contain a substantial amount of higher molecular weight components such as ethane and to a lesser extent, propane, butanes, and higher components. The ethane and heavier components are of greater value as chemical feedstocks than they are as a fuel.
It has long been known to separate ethane and higher components from methane by the use of an expander wherein a natural gas feedstream is passed to a high pressure separator and the vapor taken off and passed to an expander with the resulting vapor going to the upper portion of a demethanizer and the liquid from the separator going to the lower portion of a demethanizer. Such a system is not particularly efficient, however. Accordingly, attempts have been made to improve the efficiency simply by utilizing two or more expanders in series. However, even with multiple expanders in series, such separations are still difficult. For one thing, the subsequent demethanizer, must be rather large. Also, sufficient work may not be extracted from the system by means of the expanders even with the two or more in series to be sufficient to handle all of the compression requirements and to supply all the refrigeration needs of the overall plant.